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Overview of the V4L2 driver framework
=====================================
This text documents the various structures provided by the V4L2 framework and
their relationships.
Introduction
------------
The V4L2 drivers tend to be very complex due to the complexity of the
hardware: most devices have multiple ICs, export multiple device nodes in
/dev, and create also non-V4L2 devices such as DVB, ALSA, FB, I2C and input
(IR) devices.
Especially the fact that V4L2 drivers have to setup supporting ICs to
do audio/video muxing/encoding/decoding makes it more complex than most.
Usually these ICs are connected to the main bridge driver through one or
more I2C busses, but other busses can also be used. Such devices are
called 'sub-devices'.
For a long time the framework was limited to the video_device struct for
creating V4L device nodes and video_buf for handling the video buffers
(note that this document does not discuss the video_buf framework).
This meant that all drivers had to do the setup of device instances and
connecting to sub-devices themselves. Some of this is quite complicated
to do right and many drivers never did do it correctly.
There is also a lot of common code that could never be refactored due to
the lack of a framework.
So this framework sets up the basic building blocks that all drivers
need and this same framework should make it much easier to refactor
common code into utility functions shared by all drivers.
A good example to look at as a reference is the v4l2-pci-skeleton.c
source that is available in samples/v4l/. It is a skeleton driver for
a PCI capture card, and demonstrates how to use the V4L2 driver
framework. It can be used as a template for real PCI video capture driver.
Structure of a driver
---------------------
All drivers have the following structure:
1) A struct for each device instance containing the device state.
2) A way of initializing and commanding sub-devices (if any).
3) Creating V4L2 device nodes (/dev/videoX, /dev/vbiX and /dev/radioX)
and keeping track of device-node specific data.
4) Filehandle-specific structs containing per-filehandle data;
5) video buffer handling.
This is a rough schematic of how it all relates:
.. code-block:: none
device instances
|
+-sub-device instances
|
\-V4L2 device nodes
|
\-filehandle instances
Structure of the framework
--------------------------
The framework closely resembles the driver structure: it has a v4l2_device
struct for the device instance data, a v4l2_subdev struct to refer to
sub-device instances, the video_device struct stores V4L2 device node data
and the v4l2_fh struct keeps track of filehandle instances.
The V4L2 framework also optionally integrates with the media framework. If a
driver sets the struct v4l2_device mdev field, sub-devices and video nodes
will automatically appear in the media framework as entities.
V4L2 clocks
-----------
Many subdevices, like camera sensors, TV decoders and encoders, need a clock
signal to be supplied by the system. Often this clock is supplied by the
respective bridge device. The Linux kernel provides a Common Clock Framework for
this purpose. However, it is not (yet) available on all architectures. Besides,
the nature of the multi-functional (clock, data + synchronisation, I2C control)
connection of subdevices to the system might impose special requirements on the
clock API usage. E.g. V4L2 has to support clock provider driver unregistration
while a subdevice driver is holding a reference to the clock. For these reasons
a V4L2 clock helper API has been developed and is provided to bridge and
subdevice drivers.
The API consists of two parts: two functions to register and unregister a V4L2
clock source: v4l2_clk_register() and v4l2_clk_unregister() and calls to control
a clock object, similar to the respective generic clock API calls:
v4l2_clk_get(), v4l2_clk_put(), v4l2_clk_enable(), v4l2_clk_disable(),
v4l2_clk_get_rate(), and v4l2_clk_set_rate(). Clock suppliers have to provide
clock operations that will be called when clock users invoke respective API
methods.
It is expected that once the CCF becomes available on all relevant
architectures this API will be removed.
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